How to top up plan with credit on account?

I have a credit of $20.00 sitting in my account and I am coming up to renewal on my base plan in a couple days. When I go to top-up menu it does not give me the option of using the credit on my account. Will it automatically deduct from the credit or is there something else I have to do? Thanks!

Yeah, it will automatically come off the credit you have in there because as the name of the service implies, you have to prepay before being able to use it. By having the credit in there, you're all set. Postpaid would be the reverse - you can use the service right away and then you'll be billed for the usage afterward.